Beams of energetic particles are routinely temporally modulated and/or swept in a direction by means of a rotating structure which attenuates the beam other than when it emanates from one or more apertures within the structure during a portion of the rotation of the structure. An example of such an application of beam chopping is described by McPherson et al., A new high-speed beam chopper for time-resolved X-ray studies, J. Synchrotron Rad., vol. 7, pp. 1-4 (2000), which is incorporated herein by reference. In various applications, a small opening in such a rotating structure is swept across an internal beam that has a substantial opening angle, with the effect of scanning an emergent beam of small cross-section, such as a pencil beam, across some region of solid angle. In particular, rotating hoops have been used to create flying spot x-ray beams that can be used to create x-ray backscatter images. When the rotating structure has a substantially cylindrical symmetry about its axis of rotation, the rotating structure may be referred to as a “hoop.”
In hoops used for scanning energetic beams, the source is typically disposed centrally, substantially coincident with the axis of rotation of the rotating hoop, as in Kalvas, et al., Fast slit-beam extraction and chopping for neutron generator, Rev. Sci. Instruments, vol. 77, 03B904 (2006).
For sources that produce penetrating radiation in a conical beam, a first collimator is disposed proximate to the source itself, to collimate the beam substantially into a plane (or into a fan beam with a small divergence parallel to the fan). A rotating hoop then collimates the beam in a direction tangential to the fan beam, so that a pencil beam emerges. Openings in the hoop may then be shaped as slits, circular or elliptical apertures, or other shapes, in order to collimate the beam in the direction tangential to the direction of hoop rotation. The distance between the focal spot of the source and the aperture is ideally as large as possible in order to minimize the divergence of the beam with increasing distance. However, this must be balanced with the desire to make the hoop itself as small as possible, to minimize the moment of inertia of the rotating structure and to keep the dimensions of the apparatus as small as possible
A hoop 10 typically employed in x-ray inspection is shown in FIG. 1, in cross-section taken in a plane transverse to axis of rotation 11. Hoop 10 has three apertures 12 located in rim 13, and creates a beam (not shown) that can scan over an approximately 86-degree field of view (FOV) 19. Penetrating radiation emanates from a source 15 of penetrating radiation, typically as Bremsstrahlung radiation from a small region of a target to which electrons from an electron gun have been accelerated. The plane 9 from which penetrating emission is emitted, such as the plane of a Bremsstrahlung target, will be referred to herein as the “effective emission plane,” and the region of emission will be referred to as the “effective beam origin” 8. The source 15 of penetrating radiation, such as an x-ray tube (as measured from the effective emission plane), is offset from the central axis 11 of the 12-inch hoop in the reverse direction (i.e. away from an object 16 being scanned) by an offset distance 17 (here, 5 inches) (i.e., away from an object 16 being scanned) in order to increase the collimation distance 18 between the x-ray focal spot in the tube and the aperture in the rim of the hoop that defines the beam. For heuristic convenience, source 15 may be referred to, herein and in any appended claims, as a “tube” or an “x-ray tube,” but it is to be understood that the invention is not limited thereby.
Rearward offset of the source relative to axis of rotation 11 of a beam-chopping hoop 10 has been required for x-ray inspection applications, because x-ray tubes that operate with a power greater than about 1.5 kW have in the past typically had focal spot sizes on the order of about 3 mm in diameter. Since smaller beam sizes are indicated for inspection purposes, the source must be sufficiently offset in the reverse direction with respect to the axis of rotation 11 of hoop 10 to maximize collimation distance 18 and allow for extinction of radiation from the perimeter of the beam so that a beam with a sufficiently “trimmed” cross section is incident on the object 16 being inspected. The beam cross section would become too large for the typical distance of five feet to the object 16 being inspected if the focal spot was positioned closer to the aperture, resulting in low-resolution, and therefore low-quality backscatter images being produced
One problem with the prior art reverse-offset hoop depicted in FIG. 1 is that the field of view 19 for a hoop with three apertures with the offset shown is limited to less than 90 degrees. Additionally, the maximum speed at which a hoop of a particular mass can be rotated is limited by stresses on the outer rim of the hoop. For example, for a hoop capable of collimating x-rays in the 180 keV range, rotation of the hoop is practically limited to about 3600 rpm. An implication of that limitation is that for a system that uses three such hoops that are interleaved temporally (i.e., only one of the three hoops produces an active x-ray beam at any given time, as in a three-sided inspection portal, for example), the image created from any one of the hoops is limited to less than 20 lines/second. A more typical hoop speed of 2600 rpm for a 225 keV system produces only 14 image lines/second. This image acquisition rate is much too slow for scanning vehicles moving more than about 5 km/hr. In the case of a hoop with six apertures rotating at 3600 rpm, while the image acquisition rate is increased from 20 to 40 lines/second, the FOV for such a reverse-offset hoop is correspondingly reduced to about 39 degrees, insufficient for many applications.
A beam chopping apparatus that would allow for faster scan rates for substantial fields of view is, therefore, highly desirable.